110 Barriers to Dispersion of River Fishes 
Every year fishes are swept down the rivers by the winter’s 
floods; and in the spring, as the spawning season approaches, 
almost every species is found working its way up the stream. 
In some cases, notably the Quinnat salmon * and the blueback 
salmon,f the length of these migrations is surprisingly great. 
To some species rapids and shallows have proved a sufficient 
barrier, and other kinds have been kept back by unfavorable 
conditions of various sorts. Streams whose waters are always 
charged with silt or sediment, as the Missouri, Arkansas, or 
Brazos, do not invite fishes; and even the occasional floods of 
red mud such as disfigure otherwise clear streams, like the Red 
River or the Colorado (of Texas), are unfavorable. Extremely 
unfavorable also is the condition which obtains in many rivers 
of the Southwest, as, for example, the Red River, the Sabine, 
and the Trinity, which are full from bank to bank in winter 
and spring, and which dwindle to mere rivulets in the autumn 
droughts. 
Favorable Waters have Most Species.—In general, those streams 
which have conditions most favorable to fish life will be found 
to contain the greatest number of species. Such streams invite 
immigration; and in them the struggle for existence is indi- 
vidual against individual, species against species, and not a 
mere struggle with hard conditions of life. Some of the condi- 
tions most favorable to the existence in any stream of a large 
number of species of fishes are the following, the most important 
of which is the one mentioned first: Connection with a large 
hydrographic basin; a warm climate; clear water; a moderate 
current; a bottom of gravel (preferably covered by a growth 
of weeds); little fluctuation during the year in the volume of 
the stream or in the character of the water. 
Limestone streams usually yield more species than streams 
flowing over sandstone, and either more than the streams of 
regions having metamorphic rocks. Sandy bottoms usually are 
not favorable to fishes. In general, glacial drift makes a suit- 
able river bottom, but the higher temperature usual in regions 
beyond the limits of the drift gives to certain Southern streams 
conditions still more favorable. These conditions are all well 
* Oncorhynchus tschawytscha Walbaum. 
+ Oncorhynchus nerka Walbaum. 
